Rather than using broth to help flavor this soup, we’ve turned to ground fenugreek, a spice found in curry powders.
1. Heat oil in saucepan over medium heat. Add onion, and sauté 3 to 4 minutes. Stir in garlic and fenugreek, and cook 1 minute more. Add potatoes, thyme sprigs, and 4 cups water; season with salt and pepper, if desired. Cover, and bring to a boil. Reduce heat to medium-low, and simmer 20 minutes. Stir in watercress leaves, and cook 3 to 4 minutes, or until watercress is wilted and tender, but still bright green.
2. Remove thyme sprigs, and purée soup with hand blender until smooth. Stir in buttermilk, if using, and warm 2 to 3 minutes, without boiling. Serve garnished with watercress sprigs and ground black pepper.